We study the mechanisms of biological membrane fusion, a ubiquitous process in cell biology. To elucidate the key intermediates of lipid bilayer rearrangements in the fusion reaction, we investigated the effects of membrane lipid composition on the rates of diverse fusion processes including Ca + + -triggered cortical exocytosis and low pH-triggered syncytia formation of baculovirus infected insect cells. Lysolipids, naturally-occurring lipids, whether added exogenously or produced in situ by phospholipase A2 treatment, were found to arrest fusion reversibly at an "activated state" between fusion-triggering and membrane merging. In both experimental systems the specific fusion trigger (H + or Ca + +) was applied in the presence of inhibiting concentrations of short- chain lysophosphatidylcholine. Next the cells and granules were washed with buffers containing lysolipid but without a trigger. No fusion or lysis was observed at this stage. Removal of the lysolipid 5-10 min later by washing with buffers lacking the fusion-trigger resulted in the initiation of fusion. The extent of fusion was comparable with samples which were not treated with lysolipids. The "trigger-independent" stage of biological fusion, arrested by lysolipids, required trigger application to contacting membranes for its development. Our results suggest that i) while different fusion processes utilize different triggers, the pivotal step involving membrane merger is trigger independent and lipid sensitive, ii) fast exocytotic processes share a common mechanism for membrane merger with slow viral fusion, and, finally, iii) diverse biological fusion reactions proceed through highly bent membrane intermediates, "stalks", which cannot form when inverted- cone-shaped lysolipids are present in the contacting leaflets. It is also intriguing to speculate that membrane phospholipids, controlled by phospholipase activity, might regulate calcium- triggered exocytosis by integrating multiple calcium pulses into a coordinated exocytotic event.